The present invention relates to a semiconductor device employing a grid, and more particularly to such a device which includes variable grid openings for controlling the device turn-off pattern.
Field terminated diode (FTD) devices, also known as gridistors, have heretofore been disclosed. Such a device can be generally described as a field effect transistor which employs the principle of centripetal striction and has a multichannel structure. In general, the device functions as a power rectifier with forward blocking capability. Briefly, in such a device, a grid is included within a power rectifier structure. An exemplary device structure includes: an outer P+ anode region; an intermediate N- base region; an outer N+ cathode region; and a P type grid structure. Further information on such a device can be found in the article of Houston et al, entitled, "A Field Terminated Diode," Vol. ED-23, August, 1976, of IEEE Transactions on Electron Devices.
Although field terminated diode devices exhibit many valuable characteristics, their use has not yet become wide spread due to several problems. One such problem is that satisfactory turn-off of the device is often difficult to achieve. During device turn-off, a lateral current flows through the grid, resulting in a voltage drop along the grid due to the lateral resistance of the grid structure. This voltage drop along the grid frequently causes grid debiasing effects in which the grid areas most remote from the point at which the bias is applied turn off last. For example, in a conventional device, where the bias is applied to the grid through an electrical contact point at its peripheral or outer regions, the debiasing causes the outer grid regions to turn off first and the center of the device to turn off last. This type of nonuniform turn-off pattern or characteristic is undesirable for many applications. This nonuniform turn-off pattern, especially center turn-off last, may create current and heat concentrations in the device which result in device failure.